A The present invention concerns a vacuum pump as defined in the preamble of claim 1.
In particular, the invention concerns a new type of vacuum pump representing major advantages in relation to the existing pumps which are at present available on the market and which function in a pressure range comprised between 10xe2x88x922 mbar and 10 mbar, according to a principle which is completely different from the one upon which the operation of the existing pumps for said pressure range is based.
The vacuum pumps which are at present available on the market and which are designed to operate in said pressure range function by means of a volumetric drive of the gas, irrespective of what mechanical device is used. It may be for example a cam pump, also known as xe2x80x9cRootxe2x80x9d pump, whose outlet is connected to the intake of a primary pump, generally a vane pump or a rotary piston pump, if one wishes to maintain a pressure in the order of magnitude of 10xe2x88x922 mbar to 10 mbar in a vacuum chamber or on the outlet of a molecular pump.
A xe2x80x9cRootxe2x80x9d pump is an equipment with a positive displacement which makes it possible to drive the gas at a low pressure as of the intake towards the outlet of the pump, where the pressure of the gas is higher, by means of two cams with parallel shafts rotating in a synchronised manner in the opposite sense according to a well-known principle. The tightness of such a pump is guaranteed by means of a relatively small clearance, in the order of magnitude of 0.05 mm to 0.25 mm, between the lobes of the cams and the inner wall of the pump.
Such a pump has several disadvantages, namely the following ones:
the cams and the inner wall have to be tooled very precisely, which is consequently expensive, in order to allow for the small clearances required for its tightness and a perfect adjustment of the bearings and the camshafts;
the ratio between the consumed energy and the energy which is actually required to drive the gas is relatively high, as this known pump makes it necessary to drive metal parts with a relatively high inertia and loses significant amounts of energy due to the friction at the bearings and the joints;
when the cams heat up excessively, the pump has to be stopped in order to prevent it from being damaged due to the dilatation of the cams. In order to avoid this problem, the difference in pressure between the intake and the outlet of the pump is usually restricted to 10 mbar. In practice, in order to avoid this problem, a by-pass is provided for or the pump is set in free rotation as long as the pressure is equal or superior to 10 mbar;
as each lob alternatively passes from a high-pressure zone at the outlet of the pump to a low-pressure zone at the intake of the latter, the gas is necessarily driven from the high-pressure side to the low-pressure side. The gas is adsorbed on the surface of the lobs on the high-pressure side, and a desorption of the gas takes place on the surface of the lobs when they reach the low-pressure zone of the pump, which necessarily restricts the capacity of this type of pump.
Document U.S. Pat. No. 5,295,791 concerns pumps which make it possible to compress or move a liquid according to a principle which is identical to that of the compressors described in the preamble of claim 1.
However, these pumps cannot operate at pressures which are lower than the atmospheric pressure.
One of the main aims of the present invention is to provide a vacuum pump which does not have the disadvantages of the known volumetric pumps or of the pumps described and represented in document U.S. Pat. No. 5,295,791.
Thus, the pump according to the invention is characterised in that means are provided to subject the vibrating element to a vibration having an amplitude which is at least two times and preferably a hundred times higher than the average free path between the elastic collisions of the gas particles in the chamber, whereby this average free path corresponds to the local pressure measured near said vibrating element, thus making it possible to generate, with a pressure in the chamber comprised between 10xe2x88x922 and 1000 mbar, and in particular between 0.01 and 10 mbar, sound waves forming successive compression and underpressure zones in said gas between the intake opening and the outlet opening.
The same applies to the characteristic dimensions of the path through the chamber of the intake up to the outlet of the latter, such as the hydraulic diameter at each passage, which also have to be two times and preferably a hundred times the average free path of the gas molecules flowing through said chamber, such for gas pressures comprised between 10xe2x88x922 and 1000 mbar, in particular between 0.01 mbar and 10 mbar.
Advantageously, the above-mentioned chamber has a cross section which decreases in relation to the direction of movement of the gas as of the intake opening towards the outlet opening.
According to a particularly advantageous embodiment, the above-mentioned chamber has the shape of a pavilion whose section decreases as of the intake opening of the gas up to the outlet opening of the gas.